1. Field of the Invention
The present invention relates generally to a video display device and is directed more particularly to a video display device in which a number of display cells are arranged in a 2 dimensional fashion or in an X-Y matrix form and these display cells are respectively driven by desired data to display a desired image.
2. Description of the Prior Art
Application Ser. No. 635,608 filed July 30, 1984, in which the inventors are Akio Ohkoshi, Koji Tsuruta, Hideaki Nakagawa, Satoshi Shimada entitled "Luminescent Display Cell" assigned to the assignee of the present invention and application Ser. No. 689,599 filed Jan. 8, 1985, in which the inventor is Yuji Watanabe entitled "Video Display System" assigned to the assignee of the present invention disclose background of video display devices for large displays and the disclosures of these applications are hereby incorporated by reference in the present application.
A video display device in which a number of display cells are arranged in an X-Y matrix form and these display cells are respectively driven by desired data to display a desired picture has already been proposed.
The same applicant has proposed as the display cell usable in the above video display device the following ones.
Referring to FIGS. 1 to 4, which are a front view of a luminescent display cell, a sectional view taken on line A-A thereof, a sectional view taken on line B-B thereof, and a partially cut-away perspective view of the cell. In the figures, reference numberal 1 denotes a glass envelope comprising a front panel 1A, a rear plate 1B and a side wall 1C. Within the glass envelope 1 are disposed a plurality of luminescent display segments 2 (2R, 2G, 2B), a plurality of cathodes K (K.sub.R, K.sub.G, K.sub.B) and first grids G.sub.1 (G.sub.1R, G.sub.1G, G.sub.1B) in corresponding relation to each display segment, and a common second grid (accelerating electrode) G2. The display segments 2 each comprise a phosphor layer formed on the inner surface of the front panel 1A. There are formed three display segments 2R, 2G, and 2B for the luminescence of red, green and blue, respectively. More particularly, as shown in FIG. 5, a carbon layer 3 as a conductive layer is printed in the form of a frame on the inner surface of the front panel 1A. In spaces in the frame, red, green and blue phosphor layers 2R, 2G and 2B are formed by printing as display segments so as to partially overlap the carbon layer 3. Throughout the surfaces of these phosphor layers a metal back layer 5 is formed, e.g. an aluminum layer, through a filming layer 4. Furthermore, in opposed relation to the display segments 2R, 2G and 2B comprising the above phosphor layers and inside the rear panel 1B wire cathodes K.sub.R, K.sub.G and K.sub.B are positioned, first grids G.sub.1R, G.sub.1G and G.sub.1B opposite these wire cathodes, and the second grid G.sub.2 in common to the three first grids G.sub.1R, G.sub.1G and G.sub.1B. Each wire cathode K is formed, for example, by coating the surface of a tungsten heater with carbonate as an electron emissive material. The wire cathodes K.sub.R, K.sub.G and K.sub.B are each stretched between a pair of conductive support members 6 and 7 which are disposed on both side portions of the rear panel 1B. One support member 6 is for fixing one end of each wire cathode, while the other support member 7 is provided with a spring portion 7a to which is fixed the other end of each wire cathode. According to this arrangement, and even extension of the wire cathode due to a rise of the temperature would be absorbed by the spring portion 7a, and thus the wire cathode never becomes loose. The first grids G.sub.1R, G.sub.1G and G.sub.1B are formed in a half-cylindrical shape having a cylindrical surface in corresponding relation to the wire cathodes, and a plurality of slits 8 are formed in the cylindrical surface at a predetermined pitch along the longitudinal direction of the same surface. The slits 8 are for the transmission therethrough of electrons radiated from the wire cathode K. The second grid G.sub.2 is formed with slits 9 in portions corresponding to the first grids G.sub.1R, G.sub.1G and G.sub.1B and in positions corresponding to the slits 8 of the first grids. In this case, slit portions 9R, 9G and 9B of the second grid G.sub.2 may be formed so as to have cylindrical surfaces concentric with the corresponding first grids G.sub.1R, G.sub.1G and G.sub.1B. In this construction, electron beams from the wire cathodes are radiated rectilinearly through the slits 8 and 9 of the first and second grids and are spread with respect to the longitudinal direction of the slits. On the other hand, the portions of the second grid in which are formed the slits 9 may be horizontal as shown in FIG. 6. In this case, the electron beam is radiated so that it passes through the second grid and then is curved somewhat inwardly with respect to the longitudinal direction of the slits, as shown in dotted line 30'.
On the other hand, a separator 10 formed of a conductive material is disposed to surround the display segments or elements 2R, 2G and 2B. The separator 10 not only serves as a shield for preventing a secondary electron 31 (see FIG. 6) induced by impingement of electron beam from cathode against the first or second grid G.sub.1 or G.sub.2 from rendering an adjacent display segment luminous, but also serves to form a diffusion lens which functions to spread electron beam 30 from each wire cathode K so that the electron beam is radiated throughout the corresponding display segment 2. In addition, the separator 10 is used also as power supply means for supplying a high voltage, e.g. 10 kV, to each display segment. In assembling, the separator 10 is supported between the front panel 1A and side wall 1C of the glass envelope 1 and fixed by frit. More specifically, as shown in FIG. 7, the separator 10 is in the form of a frame partitioned in threes to surround the display segments, and on first opposed upper ends thereof are formed outwardly projecting supporting pieces 11, while on the other opposed upper ends are formed anode leads 12 for the supply of high voltage (anode voltage). Furthermore, on the side portions of the separator 10 are formed outwardly bent elastic positioning pieces 13. When the separator 10 is inserted from above into the inside of side wall 1C, as shown in FIG. 8, the supporting pieces 11 abut the upper end face of the side wall 1C to thereby support the separator 10, and at the same time the bent portions 13 abut the inner surface of the side wall 1C to thereby position the separator 10 in central fashion. Also provided on the upper end portion of the separator 10 are inwardly bent lugs 14 each having a projection 15 formed on the surface thereof. When the front panel 1A is placed and sealed on the side wall 1C after enclosing the separator 10 in the side wall 1C, the projections 15 contact the carbon layer 3 or the metal back layer 5 (see FIG. 9). As a result, the high voltage from the anode leads 12 is fed in common to the display segments 2R, 2G and 2B. In an assembled state, the anode leads 12 to which is applied the high voltage are drawn out to the exterior through the sealed portion between the front panel 1A and the upper end face of the side wall 1C, while the leads of the wire cathodes K, first grid G.sub.1, and second grid G.sub.2 are drawn out to the exterior through a sealed portion between the rear plate 1B and the side wall 1C. The leads of the cathodes K, first grids G.sub.1, and second grid G.sub.2 are brought out together for supporting purposes. For example, in each of the first grids G.sub.1R, G.sub.1G and G.sub.1B, two leads on each side, namely, a total of four leads on both sides, are brought out as leads 16G.sub.1, 17G.sub.1, and 18G.sub.1 (see FIG. 4). In the case of the second grid G.sub.2, four leads 19G.sub.2 are brought out corresponding to the four corners of the rear panel. Leads 20F of the cathodes K are brought out together to the right and left from both support members 6 and 7. The leads 20F of the cathodes are connected in common for each of the support members 6 and 7. Also with respect to each of the first and second grids G.sub.1 and G.sub.2, the corresponding leads are connected in common.
The glass envelope 1 is provided by sealing the front panel 1A, side wall 1C and rear plate 1B with respect to each other by frits 22 (see FIG. 9). To the rear plate 1B is a chip-off pipe 21 for gas exhaust fixed by frits.
Operation of the above construction will now be explained. An anode voltage of, say, 10 kV or so is supplied through the anode leads 12 to the red, green and blue display segments 2R, 2G and 2B. To each of the first grids G.sub.1R, G.sub.1G and G.sub.1B is applied a voltage of, say, 0-30V, while to the second grid G.sub.2 is applied a voltage of, say, 300 V. The wire cathodes K.sub.R, K.sub.G and K.sub.B are of 60-70 mW or so per wire. In this construction, the anode side and the second grid G.sub.2 are fixed in voltage, while the voltage applied to the first grids G.sub.1 is changed to turn on and off the display segments selectively. More particularly, when OV is applied to a first grid G.sub.1, an electron beam from cathode K is cut off and the corresponding display segment 2 is not rendered luminous. When, say, 30 V is applied to a first grid G.sub.1, an electron beam from cathode K passes through the first grid G.sub.1, then is accelerated by the second grid G.sub.2 and impinges upon the phosphor of the corresponding display segment 2 to make the latter luminous. At this time, the luminance is controlled by controlling the pulse width (duration) of the voltage (30 V) applied to the first grid G.sub.1. Further, as shown in FIG. 6, the electron beam from cathode K is spread by the separator 10 and radiated to the entire surface of the display segment 2. When the electron beam from the cathode impinges upon the first and second grids, there are produced the secondary electrons 31 from these grids, but these secondary electrons are obstructed by the separator 10, so they do not impinge upon the adjacent display segment 2. In this way, by selectively controlling the voltage applied to the first grids, the display segments 2R, 2G and 2B are rendered luminous selectively at a high luminance.
This luminescent display cell 40 is constructed in thin fashion as a whole. Besides, the low voltage-side leads such as the cathode and first and second grid leads are drawn out from the rear plate 1B side of the glass envelope 1, while the high voltage-side anode leads 12 are drawn out from the front panel 1A side. Therefore, possible dangers during discharge and wiring can be avoided, thus ensuring a stable luminescent display.
Moreover, since the anode voltage-applied separator 10 surrounds each display segment 2, a diffusion lens is formed by the separator 10. Therefore, even if only the first grids G.sub.1 are curved and the second grid G.sub.2 is flat (as shown in FIG. 6), the electron beam from cathode K spreads laterally (in the direction of the slits) and is radiated to the entire surface of the display segment 2. At the same time, the secondary electron from the first or second grid is obstructed by the separator 10, so the adjacent cut-off segment is not rendered luminous.
In the case of a color display (for example, in the case of a 9300.degree. K. white picture), the luminance mixing ratio is about 7% blue, about 13% red, and about 80% green. In the case where wire cathodes are used as an electron emission source, they are in many cases used in a temperature restriction area in order to maintain their service life. And the problem of making the luminance of the green cathode higher than that of the other cathodes can be solved by increasing the number of the green cathodes used. For example, two green cathodes K.sub.G, one red cathode K.sub.R, and one blue cathode K.sub.B may be used. As a result, the total amount of electrons for green becomes larger than that for red and blue, thus making it possible to effect a color display. It goes without saying that red and blue cathodes may also be used in plural numbers, which is effective in prolonging their service life. Thus, by increasing the number of green cathodes in comparison with the other cathodes, the luminance of green can be enhanced and a good white balance is obtainable. Consequently, an excessive load is not imposed on the cathodes, that is, the life of the luminescent display cell can be prolonged. Actually, two green cathodes are disposed in spaced relation at a distance of about 0.8 to 1 mm. As to the amount of electrons emitted, an increase of 70 to 80% can be expected though it does not become twice as large as that in the case of a single green cathode due to the electron scattering effect. Alternatively, the green luminance may be enhanced by making the area of the green phosphor layer larger than of the red and blue phosphor layers.
Since the wire cathodes are used in the temperature restriction area, that is, the loading of the oxide cathode is set at a ratio of one to several tens to prevent a red-looking appearance, the amount of electrons emitted per cathode is small. One method for solving this problem may be to substantially enlarge the surface area of oxide by winding a tungsten wire spirally, for example. But, in the case of a long spiral, it is likely that there will occur loosening or vibration of the cathode. In view of this point, such a construction as shown in FIGS. 10 and 11 is suggested.
In this example, a core 35 formed of a high-temperature material such as, for example, tungsten or molybdenum, is provided and its surface is coated with an insulating material 36 such Al.sub.2 O.sub.3. Then tungsten wire 37 serving as a heater is wound spirally thereon and an electron emissive material 38, e.g. carbonate, is bonded to the spiral portion by spraying or electrodeposition to constitute a direct heating cathode 34. The core 35 is fixed at one end thereof to one support member 6 and at the other end thereof to the spring portion 7a of the other support member 7 by spot welding or other suitable means, it being stretched under tension. The tungsten wire is fixed between one support member 6 and a second support member 6' on the other side by spot welding or other suitable means.
Thus, in the above construction, the cathode is wound spirally onto the core 35 coated with the insulating material 36, and the core 35 is stretched by the spring portion, whereby problems such as shorting between spiral portions and thermal deformation of the spiral can be eliminated. Besides, the oxide surface area is substantially increased, and a uniform temperature distribution area (A) with reduced temperature difference between both ends and the center of the cathode becomes wider. As a result, the amount of electrons emitted can be increased, and as a whole, therefore, it is possible to increase the amount of allowable current per cathode. The curve I in FIG. 11 represents a temperature distribution.
Thus, the luminescent display cell is formed. In this case, since the separator supplied with the same high voltage as that applied to the display segments is positioned to surround the plural display segments, a diffusion lens is formed whereby an electron beam from the cathode is spread laterally and radiated to the entire surface of each display segment or element. Consequently, it is possible to make a display at a high luminance. Furthermore, by the presence of the separator, secondary electrodes from a control electrode or accelerating electrodes are obstructed, not rendering the adjacent cut-off display segment luminous, and thus a stable luminescent display can be effected.
When a picture display device is formed by using the above luminescent display cell, the following assembling method is taken.
That is, a plurality of the above luminescent display cells 40, for example, 6 (per column).times.4 (per row)=24 luminescent display cells are incorporated in a unit case 41 to form one unit as shown in FIG. 12.
Then, a plurality of the above units are arranged in an X-Y matrix form, for example, 7 (per column).times.5 (per row)=35 to form a block and then 5 blocks are arranged laterally to form a submodule. Then, a plurality of the submodules are combined in an X-Y matrix form, for example, 9 (per column).times.4 (per row)=36. By using a number of the submodules, a jumbo-size picture display tube of, for example, 25 m (per column).times.40 m (per row) is constructed. In this case, the number of the display cell is EQU 36.times.5.times.35.times.24=151,200
and the number of the display segment is 3 times the above number and hence about 450,000.
FIGS. 13A and 13B are respectively a front view and a cross-sectional view of whole of a built-up jumbo-size picture display device. The whole of this jumbo-size picture display device is a building which is, for example, 42 m in height and 47 m in width. The upper portion of this building is made as a display portion which is provided with 9 floors, each floor having the height of 2.688 m. On each floor there are located 4 submodules in the lateral direction. Further, on the lower portion of the building there are formed a stage for entertainment, anterooms, a central control room for operating and managing the display device and the stage and so on.
By the above way, the picture display device is built. In this case, since 24 luminescent display cells form a unit and a number of the units are employed to assemble whole the picture display device, the display device becomes easy in handling and also easy in assembling. In this case, each unit is formed of a square shape of 40 cm in both height and width in the above example.
By the way, in such picture display device, when the display signal for each display cell is transmitted, it is impossible to perform the signal transmission in parallel for about 450,000 picture segments or elements. Thus, the signal transmission is carried out by the scanning method. In this case, however, the structure of the display device is of a unit utilization type, if the known line-sequence scanning is employed, a large number of the connections between the respective units in the lateral direction is required and hence the installation work thereof becomes complicated.
Further, since the display device is a jumbo one as set forth above, if the signal transmission is carried out, in an analog fashion, there is easily caused an error such as a crosstalk, time-base error and so on. Thus, it may be considered that the signal is transmitted in the form of a digital signal. However, if a flat cable is generally used as the transmission line, the transmission speed is generally suppressed to about 300 kHz. On the other hand, the time to send the signal to whole the picture screen is limited to 1/30 second.
Further, the above display device, the brightness characteristic of each display cell tends to fluctuate. Generally, in the display cell of the mass-production type, due to the distance between the cathode and the grid, the fluctuation of the deposition of carbonate which becomes the electron emission substance or the secular variation of the carbonate and the like, the electron emission characteristic of the cathode fluctuates and hence the brightness characteristics fluctuate from one to another. Accordingly, as mentioned before, when a large number of these display cells are aligned to form a display screen, the brightness characteristics fluctuate caused by the fluctuation of the electron emission characteristics so that the whole display becomes deteriorated in quality. Therefore, even if a large number of the brightness steps can be obtained, such brightness steps become non-effective.
While it may be considered that, for example, a control or the like is provided for each display cell so as to delicately adjust the brightness thereof, when the number of the display cells becomes very large as described above, such adjustment becomes very difficult.